1. Field of the Invention
The invention relates generally to voltage controlled oscillators (VCOs) and specifically to VCO circuitry that can be completely fabricated within a semiconductor integrated circuit and that does not use a voltage-variable capacitor (varactor) for its tuning.
2. Description of the Prior Art
Astable circuits such as oscillators find many valuable uses in various kinds of electronic equipment. The simplest and most traditional of oscillators uses an inductor (L) and a capacitor (C) in an LC tank circuit with an amplifier to overcome inescapable circuit losses that would otherwise dampen any oscillations in the LC tank. The frequency of oscillation depends on the circuit inductance (inductor L plus stray inductances Ls) and the circuit capacitance (capacitor C plus stray capacitances Cs). The general formula relating frequency to inductance and capacitance is: ##EQU1## It is therefore obvious that high frequencies require smaller values of inductance and capacitance. At low frequencies, such as those used in AM radio, the stray reactances (Ls and Cs) are insignificant. But at high frequencies, such as those used for VHF television broadcasting, the stray reactances (Ls and Cs) are relatively large and a source of problems.
The frequency accuracy and stability of LC oscillators is not very good. Inductors and capacitors are basically coils of wire and plates separated by dielectric, respectively. Close tolerance devices are expensive to produce but still have accuracies not much better than one percent. Thermal and long term stability are also major problems.
More accurate frequencies are generated by crystal oscillators where the piezoelectric effect of a cut crystal is used to source a weak frequency signal that is built-up by an amplifier. Crystal oscillators have the advantage of very accurate and stable frequency operation, but are difficult to tune to other frequencies because the crystal cannot be reactively pulled off its natural frequency by more than a few fractions of a percent.
Frequency synthesis with phase locked loops (PLLs) involves a type of oscillator that has crystal oscillator accuracy and stability, and yet can be digitally tuned to a wide range of frequencies. Most modern radios, televisions, and communications equipment use frequency synthesis. A fixed frequency crystal oscillator is used in a synthesizer to generate a reference frequency. A phase detector compares the reference frequency to the output of a voltage controlled oscillator after being divided down by a digital counter. The output of the phase detector is amplified and integrated for use as a control to the VCO. The output of the VCO will lock onto a harmonic "N" of the reference frequency, where "N" is the count value of the digital divider.
Prior art VCOs have commonly employed a voltage variable capacitor (varactor) to control with a voltage the capacitance of an oscillator. The varactors are typically semiconductor devices that reverse bias a PN junction and use the fact that the depletion zone at the junction will increase with voltage and thereby decrease the electrode capacitance across the reversed bias PN junction.
In general, VCOs may be described as second order circuits. This means that prior to saturation or steady state operation, the VCO has a linear feedback circuit that may be described by a second order differential equation. The solution of this equation may be written as: EQU V(t)=A* sin (wt+.phi.)exp(-.alpha.t)
Close examination of this result shows that to build an oscillator, the constant .alpha. must be negative. When it is negative, the solution is a sinewave of exponentially growing magnitude. Any noise within the circuit (at startup) will provide a seed for oscillation to commence. The oscillations build in amplitude until transistors used in the oscillator to provide amplification start to saturate or cut-off. Then the gain attains its limits, which results in a steady output of sinusoidal oscillation having a constant amplitude.
For term .alpha. to be a negative, the equivalent admittance within the oscillator must also be negative. This can be done by using positive feedback. In a practical circuit, an amplifier is constructed with positive gain. A portion of the output of the amplifier is routed back to its input, for example, through a resistor. Given that the gain G is greater than unity and has a zero degree phase shift, the input admittance Y can be written: ##EQU2##
In practical oscillators running at frequencies above one gigahertz, any amplifier used will have some phase shift associated with its circuitry, and that unavoidably produces a phase delay, or phase-lag. Such a phase-lag in the amplifier can be compensated for in the feedback circuit by using a phase-lead circuit. Capacitors produce a phase-lead, so the feedback circuit will typically comprise a resistor-capacitor (RC) combination.
Mobile/portable radio communication units are limited in the size and weight of devices they can incorporate. Integrated circuit techniques have been used in countless applications to put all or nearly all of the circuit components on a single chip. However, inherent IC processing variations can produce troublesome deviations in resistance, capacitance or other important characteristics, particularly in sensitive circuits such as VCOs. These deviations can dramatically affect the frequency accuracy and operational noise levels of VCOs that have been incorporated into an IC. Low-noise VCOs, subject to precision high-frequency requirements, have traditionally required very tight IC processing control. Such manufacturing control makes these VCOs very expensive to produce.
A robust VCO circuit design and implementation is needed that is relatively immune to IC processing variations, and yet produces a quality, low-noise VCO that is viable at gigahertz frequencies and does not require inductors or varactors on or off the chip.